3.3.3Periodic Table and Energy
Qualitative Analysis — OCR A Level Chemistry A H432 Revision Notes & Topical Questions
5 min read 5 parts Lesson 9 of 21
Section 1 of 5Tests for Anions
Anion and cation tests use distinctive precipitates, gases and colours that identify the ions in a salt.
Carbonate
- Add dilute acid and look for effervescence; bubble the gas through limewater,
- which turns milky if it is CO₂: CO₃²⁻(aq) + 2H⁺(aq) → H₂O(l) + CO₂(g).
Sulfate
- Add dilute hydrochloric acid, then barium chloride solution; a white precipitate of BaSO₄ shows sulfate ions.
Halides
- Add dilute nitric acid, then silver nitrate solution;
- chloride is white, bromide is cream and iodide is yellow.
Order
- Test for carbonate first, then sulfate, then halide, because carbonates would otherwise interfere with the other tests.
Section 2 of 5Silver Halide Precipitates
1
Silver nitrate reacts with halide ions to form the precipitate: Ag⁺(aq) + X⁻(aq) → AgX(s).
2
Silver chloride is white and dissolves in dilute aqueous ammonia, forming the complex ion [Ag(NH₃)₂]⁺.
3
Silver bromide is cream and dissolves in concentrated aqueous ammonia but not in dilute ammonia.
4
Silver iodide is yellow and does not dissolve in either dilute or concentrated ammonia.
5
Nitric acid is added first to remove carbonate ions, which would otherwise give a false white precipitate with silver ions.
Section 3 of 5Test for Ammonium and Gases
Test for Ammonium and Gases
Ammonium ions
- Warm the sample with sodium hydroxide solution: NH₄⁺(aq) + OH⁻(aq) → NH₃(g) + H₂O(l).
Ammonia gas
- The gas turns damp red litmus paper blue and has a characteristic sharp smell.
Carbon dioxide
- Turns limewater cloudy-white because insoluble calcium carbonate forms.
Reporting results
- Always state the reagent, the observation and the conclusion, with the ionic formula of the species identified.
| Ion | Reagent | Positive result |
|---|---|---|
| CO₃²⁻ | Dilute acid | Fizzing; CO₂ turns limewater milky |
| SO₄²⁻ | Acidified BaCl₂ | White precipitate |
| Cl⁻ | Acidified AgNO₃ | White, soluble in dilute NH₃ |
| Br⁻ | Acidified AgNO₃ | Cream, soluble in conc. NH₃ |
| I⁻ | Acidified AgNO₃ | Yellow, insoluble in NH₃ |
Section 4 of 5Worked Example
Question
- A solid X gives no gas with dilute nitric acid.
- Its solution with acidified silver nitrate gives a cream precipitate that is insoluble in dilute aqueous ammonia but dissolves in concentrated ammonia.
- Identify the anion and write the ionic equation.
- No effervescence with acid shows that carbonate is absent.
- A cream precipitate is typical of silver bromide, and solubility in concentrated but not dilute ammonia confirms it.
- The anion is bromide, Br⁻, and the precipitate is formed from Ag⁺ and Br⁻.
Answer Bromide ion: Ag⁺(aq) + Br⁻(aq) → AgBr(s)
Section 5 of 5Exam Checklist & Tips
3.3.3 – Key Points for the Exam
- Precipitate An insoluble solid formed when two solutions are mixed.
- Limewater Aqueous calcium hydroxide, which turns cloudy when carbon dioxide is bubbled through it.
- Effervescence Fizzing caused by the release of a gas from a liquid.
- Complex ion A central metal ion bonded to ligands, such as [Ag(NH₃)₂]⁺.
Common Mistakes
- Adding barium chloride before removing carbonate ions, which gives a false positive for sulfate.
- Acidifying the sulfate test with sulfuric acid, which adds sulfate ions.
- Describing the silver bromide precipitate as white rather than cream.
- Writing observations such as "ammonia is formed" without the test result, for example damp red litmus turns blue.
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